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Date: Wed, 25 Mar 2015 18:29:22 +0100
From: Ingo Molnar <mingo@kernel.org>
To: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        Steven Rostedt <rostedt@goodmis.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>, Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Alexei Starovoitov <ast@plumgrid.com>, Will Drewry <wad@chromium.org>,
        Kees Cook <keescook@chromium.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/4] x86/asm/entry/64: do not TRACE_IRQS fast SYSRET64
 path
Message-ID: <20150325172922.GA7321@gmail.com>
References: <1427303896-24023-1-git-send-email-dvlasenk@redhat.com>
 <1427303896-24023-2-git-send-email-dvlasenk@redhat.com>
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* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> SYSRET code path has a small irq-off block.
> On this code path, TRACE_IRQS_ON can't be called right before interrupts
> are enabled for real, we can't clobber registers there.
> So current code does it earlier, in a safe place.
> 
> But with this, TRACE_IRQS_OFF/ON frames just two fast instructions,
> which is ridiculous: now most of irq-off block is _outside_ of the framing.
> 
> Do the same thing that we do on SYSCALL entry: do not track this irq-off block,
> it is very small to ever cause noticeable irq latency.
> 
> Be careful: make sure that "jnz int_ret_from_sys_call_irqs_off" now does
> invoke TRACE_IRQS_OFF - move int_ret_from_sys_call_irqs_off label before
> TRACE_IRQS_OFF.

> @@ -345,8 +346,8 @@ tracesys_phase2:
>   */
>  GLOBAL(int_ret_from_sys_call)
>  	DISABLE_INTERRUPTS(CLBR_NONE)
> -	TRACE_IRQS_OFF
>  int_ret_from_sys_call_irqs_off:
> +	TRACE_IRQS_OFF
>  	movl $_TIF_ALLWORK_MASK,%edi
>  	/* edi:	mask to check */

This latter trick absolutely needs a comment, to keep future lockdep 
developers from wondering about the mismatch and the weird label 
placement ...

Thanks,

	Ingo